Germ Layer

Information about Germ Layer

Organs derived from each germ layer. Image from NCBI.

A germ layer is a collection of cells, formed during animal embryogenesis. Germ layers are only really pronounced in the vertebrates. However, all animals more complex than sponges (eumetazoans and ) produce two or three primary tissue layers (sometimes called primary germ layers). Animals with radial symmetry, like cnidarians, produce two called ectoderm and endoderm, making them diploblastic. Animals with bilateral symmetry produce a third layer in-between called mesoderm, making them triploblastic. Germ layers will eventually give rise to all of an animal’s tissues and organs through a process called organogenesis.

Germ layers

Gastrulation of a diploblast: The formation of germ layers from a (1) blastula to a (2) gastrula. Some of the ectoderm cells (orange) move inward forming the endoderm (red).

Caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. Later, Heinz Christian Pander discovered germ layers while studying chick embryos.

Among animals, sponges show the simplest organization, having a single germ layer. Although they have differentiated cells (e.g. collar cells), they lack true tissue coordination. Diploblastic animals, Cnidaria and ctenophores, show an increase in complexity, having two germ layers, the endoderm and ectoderm. Diploblastic animals are organized into recognisable tissues. All higher animals (from flatworms to man) are triploblastic, having in addition a mesoderm. Triploblastic animals develop recognisable organs.

Development

Fertilization leads to the formation of a zygote. During the next stage, cleavage, mitotic cell divisions transform the zygote into a tiny ball of cells called a blastula. This early embryonic form undergoes a massive reorganization called gastrulation forming a gastrula with either two or three layers (the germ layers). In all vertebrates, these are the forerunners of all adult tissues and organs.

The appearance of the archenteron marks the onset of gastrulation.

In humans, after about three days, the zygote has formed a solid mass of cells by mitotic division, called a morula. This then changes to a blastocyst, consisting of an outer layer called a trophoblast, and an inner cell mass called the embryoblast. Filled with uterine fluid, the blastocyst breaks out of the zona pellucida undergoes implantation. The inner cell mass initially has two layers: the hypoblast and epiblast. At the end of the second week, a primitive streak appears. The epiblast in this region moves towards the primitive streak, dive down into it, ans form a new layer, called the endoderm, pushing the hypoblast out of the way (this goes on to form the amnion.) The epiblast keeps moving and forms a second layer, the mesoderm. The top layer is now called the ectoderm.

Endoderm

Main article: Endoderm

The endoderm is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.

The endoderm consists at first of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube excepting part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm). The lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas, the epithelium of the auditory tube and tympanic cavity, of the trachea, bronchi, and air cells of the lungs, of the urinary bladder and part of the urethra, and that which lines the follicles of the thyroid gland and thymus are also formed by this layer.

The endoderm forms: stomach, colon, liver, pancreas, urinary bladder; lining of urethra, epithelial parts of trachea, lungs, pharynx, thyroid, parathyroid, intestine.

Mesoderm

The mesoderm aids in the production of cardiac muscle, skeletal muscle, smooth muscle, tissues within the kidneys, and red blood cells.

Main article: Mesoderm

The mesoderm germ layer forms in the embryos of animals more complex than cnidarians, making them triploblastic. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm.

This key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals. The formation of a mesoderm led to the formation of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions protect them from shocks.

The mesoderm forms: skeletal muscle, skeleton, dermis of skin, connective tissue, urogenital system, heart, blood (lymph cells), and spleen.

Ectoderm

Main article: Ectoderm

The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.

The ectoderm forms: Central nervous system, retina and lens, cranial and sensory, ganglia and nerves, pigment cells, head connective tissue, epidermis, hair, mammary glands.

Neural crest

Due to the great importance of the neural crest it has been referred to as the fourth germ layer. It is, however, derived from the ectoderm.

References

Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.

• • [ e] Human development of embryo and development of fetus (some dates are approximate - see Carnegie stages and a timeline)
Week 1	Zygote - Morula - Blastomere - Blastula/Blastosphere/Blastocyst/Blastocoele - Inner cell mass
Week 2	Bilaminar disc (Hypoblast, Epiblast) - Archenteron/Primitive streak
Week 3 (Trilaminar embryo)	Primitive knot/Blastopore - Primitive groove - Gastrula/Gastrulation Germ layer: Ectoderm (Neuroectoderm) - Endoderm - Mesoderm (Chorda-, Paraxial, Intermediate, Lateral plate) - Splanchnopleure/Somatopleure Somite/Somitomere (Sclerotome, Myotome, Dermatome)
Uterine support	Allantois - Trophoblast (Cytotrophoblast, Syncytiotrophoblast, Gestational sac) - Yolk sac - Vitelline duct - Placenta - Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) - Amniotic sac (Amnion, Chorion) - Decidua - Decidual cells - Intervillous space - Chorionic villi
Branchial region	Branchial arch (1st/Meckel's cartilage, 2nd)/Branchial pouch - Cervical sinus - Lateral lingual swelling - Tuberculum impar - Frontonasal prominence - Olfactory pit - Nasal processes - Globular process - Primitive palate
Histogenesis and organogenesis	Glands: Thyroglossal duct Limb development: Limb bud - Apical ectodermal ridge/AER

• • [ e] Germ layers
Germ Layers	Ectoderm • Endoderm • Mesoderm
Production	Histogenesis • Organogenesis

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Embryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum, egg, which, after fertilization, is then called a zygote.
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Vertebrata
Cuvier, 1812

Classes and Clades

See below
Vertebrates are members of the subphylum Vertebrata (within the phylum Chordata), specifically, those chordates with backbones or spinal columns.
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Porifera
Grant in Todd, 1836

Classes
Calcarea
Hexactinellida
Demospongiae
The sponges or poriferans (from Latin "pore" and "to bear") are animals of the phylum Porifera. Porifera translates to "Pore-bearer".
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Eumetazoa
Butschli, 1910

subgroups

Placozoa
Radiata
Bilateria

Eumetazoa is a clade comprising all major animal groups except sponges.
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Cnidaria
Hatschek, 1888

Subphylum/Classes^[1]

Anthozoa — corals and sea anemones

Medusozoa:^[2]

:Cubozoa — sea wasps or box jellyfish

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The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.

What forms from it (general)?

Nervous system
Outer part of integument

What forms from it (vertebrates)?

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Endoderm is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm.

The endoderm consists at first of flattened cells, which subsequently become columnar.
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Diploblasty is a condition of the ovum in which there are two primary germ layers: the ectoderm and endoderm.

Diploblastic organisms are organisms which evolve from such an ovum, and include cnidaria and ctenophores. The endoderm allows them to develop true tissue.
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The germ layer mesoderm forms in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms during gastrulation when some of the cells migrating inward to form the endoderm form an additional layer between the endoderm and the ectoderm.
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Triploblasty is a condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Additionally, the term may refer to any ovum in which the blastoderm splits into three layers.
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Biological tissue is a collection of interconnected cells that perform a similar function within an organism.

The study of tissue is known as histology, or, in connection with disease, histopathology.
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organ (Latin: organum, "instrument, tool") is a group of tissues that perform a specific function or group of functions. Usually there is a main tissue and sporadic tissues. The main tissue is the one that is unique for the specific organ.
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In animal development, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero.
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Caspar Friedrich Wolff (January 18, 1733 in Berlin, Germany - February 22, 1794 in St.Petersburg, Russia) is one of the founders of embryology. In 1759 Wolff graduated as an M.D.
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Heinz Christian Pander (1794-1865) was a Russian biologist. Pander studied the chick embryo and discovered the germ layers (i.e., three distinct regions of the embryo that five rise to the specific organ system).

References

Gilbert, Developmental biology

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Choanocytes (also known as "collar cells") are cells that line the spongocoel of Ascinoid sponges that contain a central flagellum surrounded by a collar of microvilli. It is the closest family member to the free-living ancestor called choanoflagellate.
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Cnidaria
Hatschek, 1888

Subphylum/Classes^[1]

Anthozoa — corals and sea anemones

Medusozoa:^[2]

:Cubozoa — sea wasps or box jellyfish

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Ctenophora
Eschscholtz, 1829

Classes

Tentaculata
Nuda

The phylum Ctenophora, commonly known as Comb Jellies, is a phylum classically grouped with Cnidaria in the Coelenterata infrakingdom.
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The ectoderm is the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.

What forms from it (general)?

Nervous system
Outer part of integument

What forms from it (vertebrates)?

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Fertilization (also known as conception, fecundation and syngamy), is fusion of gametes to form a new organism of the same species. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo.
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For other meanings see Zygote (disambiguation).

A zygote (Greek: ζυγωτόν) is a cell that is the result of fertilization.
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In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote.
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Germ Layer

Information about Germ Layer

Germ layers

Development

Endoderm

Mesoderm

Ectoderm

Neural crest

References

See also

What forms from it (general)?

What forms from it (vertebrates)?

References

What forms from it (general)?

What forms from it (vertebrates)?